pulsar timing

A pulsar ticks like a clock — and like any good clock, the real value is not the tick itself but in what you notice when the ticking is the tiniest bit early or late. Pulsar timing is the art of recording a pulsar's pulses over months and years with extraordinary precision, then watching for deviations from perfect regularity. Because the cleanest pulsars are as steady as atomic clocks, the smallest disturbance — a tug of gravity, a passing planet, a ripple in spacetime — leaves a fingerprint in the arrival times.

Here is how it works. Astronomers predict exactly when each pulse should arrive, accounting for everything they can: Earth's motion, the pulsar's slow spin-down, the gas the signal crosses. Then they compare prediction with reality, pulse by pulse, to nanosecond accuracy. Any leftover discrepancy must come from something unmodelled. This technique has weighed pulsars and their companions, discovered the first planets ever found outside our solar system (around pulsar PSR B1257+12 in 1992), and confirmed Einstein's prediction that orbiting masses radiate gravitational waves.

The grandest application is the pulsar timing array: monitoring dozens of millisecond pulsars across the sky at once, turning the whole galaxy into a gravitational-wave detector light-years wide. As a long, slow gravitational wave from distant merging supermassive black holes washes through the galaxy, it nudges the pulse arrival times in a correlated pattern. In 2023 several teams reported the first evidence of exactly this hum — a 'background' of gravitational waves — though the signal is faint and still being confirmed.